Process for Coating Discrete Articles with a Zinc-Based Alloyed Layer

ABSTRACT

The present disclosure concerns a process suitable for coating discrete articles with a zincrich, fully alloyed layer. A known method for the corrosion-protection of such articles comprises the steps of hot-dip galvannealing, typically followed by painting. This hot-dip process has however to be performed at a high temperature, thereby submitting the articles to severe thermal stress. 
     A novel vacuum deposition process of Zn is therefore presented. It is characterized in that, in the step of contacting the article with metallic Zn vapor, the temperature of the article is equal to or higher than the dew point of the Zn vapor. The process results in a coating having a uniform thickness, even on less accessible surfaces. The surface roughness is well adapted for the adhesion of paint.

This application is a national stage application of InternationalApplication No. PCT/EP2010/000684, filed Feb. 4, 2010, which is acontinuation-in-part of International Application No. PCT/EP2009/000750,filed Feb. 4, 2009, the entire contents of these applications are herebyincorporated herein by reference.

The present disclosure concerns a process suitable for coating articles,and in particular discrete articles, with a zinc-rich, completelyalloyed layer.

By discrete articles are meant non-continuous articles, typically havingat least one concave surface. They often comprise an assembly ofconnected parts.

The disclosed process is suitable for applying a zinc-based protectivecoating on iron or steel, whereby Zn—Fe intermetallics are formed acrossthe full thickness of the coating. This coating is similar to the layerresulting from the so-called “galvannealing” process. It differs fromgalvanized layers, which have Fe-free Zn at their outer surface.

A surface consisting of Zn—Fe intermetallics is preferred to a Znsurface when painting of the substrate is envisaged. It indeed offers asuperior long-term paint adhesion and an excellent corrosion resistanceat the interface between the paint and the Zn-bearing layer. Anotheradvantage is the good spot welding behavior, which is important for theautomotive market. However, the limited ductility of the layer should betaken into account if the product has to be further fashioned, as it istypically the case for continuous products.

In order to produce a zinc-rich, completely alloyed layer, continuousproducts such as sheets and wires are usually galvannealed by re-heatingshortly a previously galvanized surface above the melting temperature ofzinc.

JP-A-58034167 describes a typical process, whereby the continuousproduct is galvanized using hot-dipping in a molten Zn bath at about465° C. When drawn out of the bath, extraneous liquid zinc on top of thegalvanized layer is blown away using so called air knifes. Then, thesurface is rapidly heated to up to 600° C. and kept for some time atelevated temperature so as to complete the annealing process.

According to another process divulged in JP-A-2194162, the product isgalvanized in a vacuum-deposition station. A well defined quantity of Znis deposited on a relatively cold steel substrate at a temperature of100 to 300° C. Because of the short processing time of a few secondsonly, and of the relatively low temperature of the steel, the Zndeposition mechanism is based on condensation. The galvanized productthen passes a heating station for annealing to take place.

JP-A-59083765 concerns a continuous vacuum deposition process forgalvanizing steel sheet. The temperature of the sheet is herebymaintained below 300° C., preferably below 200° C., in order to avoidthe re-evaporation of zinc. The process is aimed at zinc plating,whereby zinc crystals are observed on the surface. The formation ofZn—Fe is not mentioned: the low processing temperatures and therelatively short residence times as normally used in continuous platinglogically exclude the formation of Zn—Fe alloys.

JP-A-63004057 also concerns a continuous vacuum deposition process forgalvanizing steel sheet. A 2-step process is described. A first step iscarried out in a vacuum deposition chamber where Zn condensates on thesheet. Besides the condensation heat, additional heating is provided tothe sheet by a winding roll. Zn—Fe alloy is then formed in a secondstep, which is carried out in the sheet exit chamber. This documentagain teaches physical condensation of Zn, as the reactive conditionsfor the formation of alloy are only reached afterwards.

The above processes can only be performed on continuous products havinga simple geometry, such as sheets and wires. For discrete products, abatch process is used.

A completely alloyed surface can be produced on discrete products in asingle step, by hotdipping in a Zn bath at a relatively high temperatureof 560 to 630° C. As Zn is particularly fluid at this temperature, thenatural flow off when extracting the articles from the bath suffices toeliminate extraneous surface Zn. Nevertheless, articles are sometimecentrifuged to accelerate Zn removal. The high temperature promotes theformation of Zn—Fe intermetallics across the full thickness of thecoating.

However, hot dipping at such high temperatures induces potentiallydeleterious thermal stress in the articles. Moreover, thecharacteristics of the steel itself can be adversely affected. Thisproblem is compounded by the fact that one typically hot dips a rackcarrying a multitude of diverse articles, made out of different gradesof steel. It then becomes impossible to define process parameters, suchas bath temperature or dipping time, suitable for all articles.

The batch process according to the present invention provides anenhanced alternative to galvannealing. A uniform intermetallic coatingthickness is obtained, even on articles made of different steel gradesor having a complex shape. Also, the problem of the induced thermalstress is largely avoided, thanks to the inherently slower and morehomogeneous heating process.

The disclosed process for coating iron or steel articles with a Zn—Feintermetallic layer comprises the steps of:

-   -   providing a sealable furnace, comprising a process chamber        equipped with heating means, means for introducing and        extracting gasses, and access ports for the article to be        coated;    -   taking the article to be coated into the process chamber;    -   contacting the article at a temperature of 200 to 650° C. with a        reducing gas in the process chamber, thereby removing surface        oxidation;    -   extracting gasses from the process chamber to a residual        pressure of less than 1000 Pa, and preferably of less than 100        Pa;    -   contacting the article at a temperature of 225 to 650° C. with        metallic Zn vapor in the process chamber, thereby coating the        article with a Zn—Fe intermetallic layer;    -   retrieving the coated article from the process chamber.

It is further characterized in that, in the step of contacting thearticle with metallic Zn vapor, the temperature of the article is,preferably permanently during this step, equal to or higher than the dewpoint of the Zn vapor.

By dew point of the Zn vapor is meant the temperature at which theambient partial pressure of Zn would condensate. The dew point can bederived from the partial pressure using known tables. Theabove-mentioned condition can e.g. be ensured in practice by providing acold zone or cold finger in the coating reactor. By cold is meant atemperature so controlled as to be slightly below the temperature of thesteel article to be coated.

In a preferred embodiment, in the step of contacting the article withmetallic Zn vapor, the temperature of the article can be equal to orhigher than the temperature of the Zn vapor. This relationship oftemperatures will prevent Zn from condensing on the article.

The needed reducing conditions can advantageously be obtained by using areducing gas, such as a mixture of N₂ and H₂. An article temperature of350 to 550° C. is preferred.

In the step of contacting with metallic Zn vapor, an article temperatureof 350 to 550° C. is preferred. The partial Zn partial pressure shouldadvantageously be in the range of 1 to 500 Pa, the upper limit beingdetermined according to the temperature of the article, and inparticular so as to avoid any condensation. Higher temperatures andhigher Zn partial pressures lead to faster layer growth.

The obtained products can usefully be painted. The Zn—Fe intermetalliclayer offers the needed roughness to guarantee a good adherence of thepaint.

Normally, articles undergo a preliminary surface preparation beforeentering the coating furnace. Articles are indeed often covered byoxides, from the steel hot rolling process or from their manufacturingprocesses. Generally, the treatment to remove this layer consists inacid pickling or shot blasting. This is performed in known ways, indedicated apparatus.

After this step, the surface is still covered by a thin layer of nativeoxides a few nanometers thick, due to air oxidation at room temperature.According to the present invention, the remaining oxides are reduced ina step performed within the coating furnace. This step aims atactivating the reactivity of the surface towards the zinc vapor.

In the reducing gas contacting process, an article temperature of 200°C. or more is needed to ensure sufficiently fast reduction kinetics. Forinstance, this step can be performed at atmospheric pressure in a N₂/H₂mixture in static conditions. The reduction can also be performed at lowpressure, e.g. between 100 and 1000 Pa, under fast flowing gasconditions. Underpressure is useful to guarantee that no H₂ escapes fromthe furnace; overpressure will enhance the reduction kinetics. Anarticle temperature of 350 to 550° C. is preferred.

In the Zn contacting process, an article temperature of 225° C. or moreis needed to allow for the formation of Zn—Fe intermetallics.Temperatures of 350 to 550° C. are preferred, as they ensure asufficiently fast diffusion of Fe through the layer while preserving thearticle from any thermal degradation.

Temperatures above 650° C., either in the process of contacting with areducing gas or with Zn vapor, are detrimental to the economy of theprocess or will often lead to the thermal degradation of the articles.

Pre-heating the article before entering the coating furnace, and havingthe article cool down after retrieving it from the coating furnace,could shorten the process time in the vacuum furnace.

When dealing with articles having carbon or organic residues on theirsurface, a preliminary oxidation step with an O₂ containing gas could beconducted in the coating furnace.

It is believed that the deposition mechanism of Zn is not condensation,but rather reactive deposition. The Zn vapor reacts directly withsurface Fe, thereby forming Zn—Fe intermetallics. The Zn—Fe phase istypically solid at the envisaged operating temperature.

Also, the Zn is trapped in a stable compound. This means that there isno risk of drippage on the surface of the articles. Due to therelatively long residence time and to the high temperature of thearticle and of its surface, Fe and Zn tend to migrate through theintermetallic layer during the exposure to Zn. As the thickness of thealloyed layer increases, the diffusion of Fe through the layer slowsdown, results in a reduced reactivity of the surface towards the Znvapor. This effect favors the growth of a layer with a uniform 30thickness all over the part to be coated. Layers of up to 100 μm can begrown. An advantage of the present process is that the Sandelin effect,which deteriorates the control of the growth of intermetallic Fe—Zncompounds on Si and P bearing steels during hot dipping, is totallyavoided. This effect occurs at moderate temperatures and is due to theformation of ζ(FeZn₁₃) filaments. It is assumed that the absence of anyliquid Zn in the present process explains this behavior.

This process is particularly well suited for coating articles of complexshape. By this are meant articles having at least one concave surfaceand/or a variable cross section about all axes. Such articles alsotypically have regions with a thickness of more than 10 mm and/orconsist of an assembly of welded parts. They often have less accessibleregions such as the inner surface of tubes.

Referring to FIG. 1, the coating furnace essentially comprises:

-   -   a gas-tight sealable process chamber (1);    -   a heating device (2) to control the temperature of the articles,        but also of the chamber's atmosphere and walls; this device        could be inside or around the process chamber;    -   a vacuum system (3), in order to extract gases such as N₂, H₂,        H₂O, and air;    -   gas injection means (4) for gases such as N₂, H₂, and air.    -   access ports (5) for introducing and retrieving the articles to        be treated;    -   a provision (6) to introduce Zn in the process chamber; either        the metal is brought directly into the chamber, or it is        introduced through gas injectors connected to evaporators.

The following example illustrates the invention.

This example concerns the deposition of Zn—Fe intermetallics and Zn onhot rolled steel plates. To this end, two 100 mm by 200 mm by 3 mm steelplates are installed close to each other in the process chamber, with agap of 10 mm between their parallel surfaces. This layout thus defines 2outer surfaces and 2 inner surfaces, thereby simulating the differencein accessibility of surfaces on real-world, complex articles.

The following steps are performed.

Step 1: Cleaning the hot-rolled the steel samples by shot blasting, inorder to remove the iron oxide layer formed in the hot rolling process.

Step 2: Introduction of samples are introduction in the coater. Thecoater comprises a treatment chamber (diameter 0.2 m, length 1 m)surrounded by an electrical resistance furnace (100 kW) providinghomogeneous heating. This assembly resides in a vacuum chamber (1 m³).40 g of Zn is introduced in an evaporator located at the bottom of thecoater.

Step 3: Vacuum suction to 0.1 mbar and introduction of reducing gases inthe process chamber (5% H₂ and N₂ 95%; dew point: −30° C.; temperature:450° C.; pressure: 0.8 bar).

Step 4: Heating of the coater and samples to 450° C. at 10° C./min.

Step 5: Reduction of the surface oxide for 600 s in the reducing gas.

Step 6: Vacuum suction to 0.03 mbar and temperature homogenization at450° C.

Step 7: Heating of the Zn evaporator to 450° C. and stabilization for 20minutes.

Step 8: Increasing the pressure to atmospheric, using air.

Step 9: Cooling of process chamber and samples to room temperature at10° C./min.

Step 10: Opening of the coater and extraction of the coated steelsamples.

It appears that the samples are coated on each surface, including thesaid inner surfaces, with a homogeneous layer formed by 50 μm of Zn—Feintermetallics.

1. A process for coating an iron or steel article with a Zn—Feintermetallic layer, comprising: providing a sealable furnace,comprising a process chamber equipped with a heating component, acomponent for introducing and extracting gasses, and access ports forthe article to be coated; taking the article to be coated into theprocess chamber; contacting the article at a temperature of 200 to 650°C. with a reducing gas in the process chamber, thereby removing surfaceoxidation; extracting gasses from the process chamber to a residualpressure of less than 1000 Pa; contacting the article at a temperatureof 225 to 650° C. with metallic Zn vapor in the process chamber, therebycoating the article with a Zn—Fe intermetallic layer; retrieving thecoated article from the process chamber; wherein when contacting thearticle with metallic Zn vapor, the temperature of the article is equalto or higher than the dew point of the Zn vapor.
 2. The process of claim1, wherein when contacting the article with metallic Zn vapor, thetemperature of the article is equal to or higher than the temperature ofthe Zn vapor.
 3. The process of claim 1, wherein the reducing gascomprises H₂.
 4. The process of claim 3, wherein the reducing gascomprises a N₂/H₂ mixture.
 5. The process of claim 1, wherein whencontacting the reducing gas, the article is at a temperature of 350 to550° C.
 6. The process of claim 1, wherein when contacting with metallicZn vapor, the article is at a temperature of 350 to 550° C.
 7. Theprocess of claim 1, wherein after retrieving the coated article, thearticle is painted.